The Growing Case for Geoengineering

David Mitchell pulls into the parking lot of the Desert Research Institute, an environmental science outpost of the University of Nevada, perched in the dry red hills above Reno. The campus stares over the tops of the downtown casinos into the snow-buried Pine Nut Mountains. On this morning, wispy cirrus clouds draw long lines above the range.Mitchell, a lanky, soft-spoken atmospheric physicist, believes these frigid clouds in the upper troposphere may offer one of our best fallback plans for combating climate change. The tiny ice crystals in cirrus clouds cast thermal radiation back against the surface of the earth, trapping heat like a blanket—or, more to the point, like carbon dioxide. But Mitchell, an associate research professor at the institute, thinks there might be a way to counteract the effects of these clouds.It would work like this: Fleets of large drones would crisscross the upper latitudes of the globe during winter months, sprinkling the skies with tons of extremely fine dust-like materials every year. If Mitchell is right, this would produce larger ice crystals than normal, creating thinner cirrus clouds that dissipate faster. “That would allow more radiation into space, cooling the earth,” Mitchell says. Done on a large enough scale, this “cloud seeding” could ease global temperatures by as much as 1.4 °C, more than the planet has warmed since the Industrial Revolution, according to a separate Yale study.